1. Field of the Invention
This invention relates to a semiconductor device having TFTs (thin film transistors) provided on an insulating substrate of glass or the like, and a method for producing the semiconductor device.
2. Description of Related Art
TFTs have been conventionally formed on a glass substrate to form a semiconductor device such as an active matrix liquid crystal device or an image sensor. The TFTs are used, for example, to drive the pixels of the liquid crystal device.
The TFTs used in the above devices are generally formed of a silicon semiconductor layer in the form of a thin film. The silicon semiconductor of a thin-film type is classified into two types, an amorphous silicon semiconductor (a-Si) type and a crystalline silicon semiconductor type. The amorphous silicon semiconductor can be relatively easily produced at a low film-forming temperature by a vapor-phase deposition method. Therefore, this type is suitable for mass production, and it has been most generally used. However, this type of silicon semiconductor has inferior physical properties such as electrical conductivity, etc. to the crystalline silicon semiconductor. Therefore, in order to more improve a high-speed response characteristic of TFTs, a producing method for TFTs comprising crystalline silicon semiconductor has been strongly required to be established. As the silicon semiconductor having crystallinity have been known polycrystalline silicon, microcrystalline silicon, amorphous silicon containing crystal components, semi-amorphous silicon having an intermediate state between crystallinity and amorphousness, etc.
The following methods may be used to obtain thin film silicon semiconductors having the foregoing crystallinity:
(1) Crystallinity is established during the formation of the semiconductor film.
(2) An amorphous semiconductor film is formed in advance, and then a laser beam is irradiated to the film to crystalize the film.
(3) An amorphous semiconductor film is formed in advance, and then heated to crystalize the film.
However, in the method (1), it is technically difficult to form a film having excellent semiconductor physical properties on the whole surface of a substrate uniformly. In addition, the film formation must be performed at a temperature above 600.degree. C. and thus an inexpensive glass substrate is unusable, so that a manufacturing cost is increased.
In the method (2), an excimer laser is most generally used at present as a laser beam source for irradiating a laser beam to an amorphous semiconductor film. In this case, the irradiation area of the laser beam is small, and thus this method has a disadvantage that a throughput is low. In addition, the stability of the laser beam is insufficient, so that the whole surface of a large-area substrate cannot be treated uniformly. That is, this method is not practically usable at present.
As compared to the methods (1) and (2), the method (3) has an advantage that it is more suitable to manufacture a large-area semiconductor film. However, this method requires a heating temperature above 600.degree. C., and thus an inexpensive glass substrate is not usable. Therefore, this method must be developed to reduce the heating temperature. Particularly in case of present liquid crystal display devices, a large-area screen design is being promoted, and thus use of a large-size glass substrate is required. When a large-size glass substrate is used, contraction and distortion of a substrate occur in a heating process which is indispensable to produce semiconductors, and they cause a critical problem that the precision of a masking process is reduced. Particularly in a case of 7059 glass which is most generally used at present, it has a distortion point of 593.degree. C., and it is greatly deformed in a conventional heat crystallization method. In addition to the heat problem as described above, a heating time required for crystallization is over several tens hours in a present process, and thus the heating time must be shortened.